This invention relates to base stations for use in cellular communications systems.
Cellular communications systems are currently in use providing radio telecommunications to mobile users. Such systems divide a geographic area into cells, each cell being served by a base station through which subscriber stations communicate. Cells are often divided into sectors with each sector being served by an antenna arrangement mounted at the base station. Sectored systems can provide increased capacity and reduced interference compared with non-sectored systems. FIG. 1 shows a typical array of cells 10, each cell being divided into three sectors 11, 12, 13 and served by a base station 14.
To meet increasing demand for mobile communications services there is interest in further improving the capacity of systems.
One known technique for improving the capacity or coverage on the uplink path of a cell site is to form fixed receive beams at the base station such that each cell sector is covered by a number of beams rather than just a single beam. By narrowing an antenna""s beam pattern in azimuth, the antenna gives increased gain in the boresight direction. For example, increasing the number of beams in a 120xc2x0 sector from 1 to N (N=4 is a suitable example), allows one to design beams giving approx. 10 log10(N) dB of gain in their boresight direction. This narrowing of the beam pattern also improves spatial filtering by rejecting interference caused by other users within the same sector (but not in the beam direction) and from users in neighbouring cells.
The combination of increased gain and reduced interference level allows for a greater path loss figure in the power budget for the uplink, and hence a greater cell range. Alternatively, for a given cell radius it is possible to increase capacity. In a typical mobile Code Division Multiple Access (CDMA) system, forming extra beams on the uplink is effectively equivalent to increasing the sectorisation factor. As an example, providing four beams per uplink sector in a tri-sectored cell gives equivalent performance gains to using cells which are divided into twelve sectors.
The simplest way to form these beams is by using separate antennas, one for each beam. Each beam is constructed as a separate antenna, such as a flat plate antenna construction with printed elements and appropriate phasing connections to provide the required directivity and hence gain. Base station antennas are normally constructed with a narrow gain pattern in elevation. This would require a tall antenna of the order of 10 to 20 wavelengths in height. Forming beams with individual passive antennas is attractive because it allows the gain pattern to be tailored to requirements. However, a beam pattern which is narrow in azimuth also requires a wide antenna aperture of several wavelengths in width. This may lead to antennas which are excessively heavy and which have a high wind loading.
An alternative technique for generating N beams with full sector coverage is to generate orthogonal beam outputs from the same aperture. The beams are orthogonal in the sense that there is zero mutual coupling between beam ports, and the average value of the cross-product of the radiation pattern of one beam with the conjugate of any other beam is zero. As an example, four beams can be generated from four radiating elements, and it is only required to support a single such antenna for each sector because the set of beams use a single common antenna aperture. A common technique for doing this beamforming is to pass antenna element outputs through passive phase shifters to create beamformed outputs in the frequency band on which the signals are received (i.e. xe2x80x98at RFxe2x80x99). One such implementation is known as the xe2x80x98Butler Matrixxe2x80x99. In order to ensure the full gain (approx. 10 log10(N) dB) at the beam peaks, phase shifters with zero attenuation (a so-called xe2x80x98uniform aperture distributionxe2x80x99) are used. This gives a number of beams with approximately a xe2x80x98sinx/xxe2x80x99 gain profile.
FIG. 2 shows a typical coverage pattern for this type of antenna structure.
Four individual beams 101, 102, 103, 104 area shown by dashed lines. The maximum gain (approx. 10 log10(N)) occurs at the beam peaks 110. The problem is that the gain of neighbouring beams has dropped by 4dB at the beam crossovers 115. These beam crossovers are halfway in angle to the first null. This is because for orthogonal beams the boresight of one beam corresponds to the null of another. These crossover points are often referred to as xe2x80x98cuspsxe2x80x99.
Cusps cause problems when attempting to provide an even cellular coverage over a certain geographical area. Mapping the locus of the cell edge, i.e. the locus of points with, on average, equal quality of service, gives the sort of xe2x80x98flower petalxe2x80x99 arrangement shown in FIG. 2. This diagram represents a single 120xc2x0 sector of a tri-sectored cell site, with 4 orthogonal beams in the sector. The cusp depth 130 in terms of power in this example is 4 dB. The geographical distance this represents i.e. the difference in cell radius between beam peak and beam cusp depends on the propagation law which in turn depends on such factors as carrier frequency and antenna heights. For a typical propagation law of 35 dB increase in path loss per decade of range increase, and for a typical cell radius (at the beam peak) of 5 km, this represents a reduction in radius at the beam cusps of around 1.2 km, giving a cell radius of 3.84 km at the cusps.
It is not simple to tessellate such cells to allow the beam peaks from one cell to coincide with the cusps from another. If the cells are tessellated as if they were circular with a 5 km radius, then there will be areas of poor availability, where the received signal quality is likely to be poor. An alternative is to treat the cells as being circular with the lesser 3.84 km radius at the cusps. This improves availability but makes inefficient use of base stations, requiring almost 70% more base stations than for 5 km radius cells to cover a given geographical area. Operators may be tempted to tessellate bases with a cell radius somewhere between 3.84 km and 5 km, but this would lead to some areas on the cell edge of above-average availability, and other areas with below-average availability.
One solution to the cusping problem is described in European Patent Application EP 0 647 978 A2. An output of a transceiver is split into two signals which are fed to two adjacent beams. This application also describes how ripple in the inter-facet region of the radiation pattern of a muti-faceted antenna can be minimised by varying the relative phase of the facets.
The present invention seeks to minimise the effects of cusping in cellular radio systems.
A first aspect of the present invention provides a method of operating a base station of a cellular communications system comprising:
forming a plurality of adjacent beams in azimuth across a coverage area, and
varying the position of the plurality of beams in unison whereby to provide a mean antenna gain in all azimuthal directions across the coverage area.
Varying the position of the beams has the effect of varying the position of the cusped regions of the beam pattern thereby reducing the effects of cusping loss across the coverage area. The position of the beams can be varied by a movement in azimuth over one half, or multiples of one half, of the angular separation of the formed beams.
Preferably there are a plurality of base stations in the system, each of whose plurality of beams are varied in position independently of the other base stations. Independently steering the beam pattern of each base station has the advantage that there is minimal correlation between the gain profile of signals received by a subscriber from adjacent base stations, or in signals received by adjacent base stations from a particular subscriber. This further minimises the effects of cusping loss.
The position of the plurality of beams can be varied by mechanically moving the antenna array. Alternatively, and more preferably, the position of the plurality of beams can be varied by electrically steering the beams by applying a phase shift to elements in the antenna array. The phase shift can take the form of a phase-shift gradient which is applied across the elements of the antenna array.
Preferably the beams are varied at a rate which is substantially equal to the rate of variation of one of the effects normally experienced by a terminal, and which the system operator incorporates a margin to accommodate.
In planning a system, a system operator uses a signal link budget to guarantee a particular quality of service to a subscriber. The link budget includes positive gain factors such as transmit power and antenna gain and negative factors such as propagation loss and margins to cope with effects such as shadowing and fading that a mobile will experience. Shadowing is typically experienced by a mobile terminal due to terrain and obstacles in the signal path between the base station and mobile.
By varying the position of the beam pattern formed by the base station, the mean antenna gain in all directions is increased, with the antenna gain at a particular point varying between a minimum gain (at the cusp) and a maximum gain (at a beam peak) as the beam pattern is moved. The link budget therefore gains several dBs due to the increased mean antenna gain, but some margin needs to be allowed in the link budget to guarantee a particular quality of service in the presence of the moving beam pattern.
A signal between a mobile and a base station will vary according to the sum of a first varying component due to movement of the beam pattern, and other varying components due to the propagation effects of shadowing. If the variation in signal level due to the beam movement is similar to the effect of shadowing then the sum, in the dB domain, of these varying components results in a received signal which has a marginally greater degree of variance compared to each effect taken alone. The overall margin which must be used in the link budget to accommodate for the effects of the beam movement and shadowing, and to guarantee a particular quality of service, is greater than the margin that the operator would have allowed for shadowing alone. However, the difference between this new overall margin and the original margin that the operator would have allowed for shadowing is less than the improvement in the link budget that is achieved by having the mean gain profile equal in all directions, therefore resulting in a net gain in the link budget. This has the advantages of allowing a larger cell for a given transmit power.
The rate at which the position of the beams is varied can be made substantially equal to the rate at which shadowing varies for a typical mobile terminal. This can be taken as the rate at which a typical mobile moves between extremes of shadowing, which is typically of the order of 5-100 s, corresponding to a required rate of beam movement of 0.01-0.2 Hz.
The position of the beams can be varied at a linear rate or pseudorandomly, with the pseudorandom variation having a time constant substantially equal to the rate at which a typical mobile terminal moves between extremes of shadowing.
In a further embodiment, the position of the beams is varied at a faster rate, which is of a similar order to the rate at which fast-fading occurs, typically 1-100 Hz. There is an upper limit to the rate at which the beam position can be varied which is due to the design constraints of a mobile terminal receiver. Mobile receivers are designed to cope with a limited rate of variation in amplitude and phase of an incoming signal.
The variation in the position of the plurality of beams can be applied to beams providing a downlink path to a terminal, to beams providing an uplink path from a terminal or to both of these.
The method is particularly suitable for a base station which operates according to a code division multiple access (CDMA) protocol.
Another aspect of the present invention provides a cellular communications base station comprising:
an antenna array which forms a plurality of adjacent beams in azimuth across a coverage area; and
a control device for varying the position of the plurality of beams in unison whereby to provide a mean antenna gain in all azimuthal directions across the coverage area.
A further aspect of the present invention provides a cellular communications system comprising at least one base station as above.
Preferred features may be combined as appropriate, and may be combined with any of the aspects of the invention, as would be apparent to a person skilled in the art.